What is adsorption?

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (the adsorbate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. The term sorption encompasses both processes, while desorption is the reverse process.

Adsorption

Adsorption is present in many natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, synthetic resins, and water purification. Adsorption, ion exchange, and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column.

Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding).

Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials.

The first mathematical fit to an isotherm was published by Freundlich and Küster (1894) and is a purely empirical formula for gaseous adsorbates,

<math>\frac{x}{m}=kP^{\frac{1}{n}}</math>

where <math>x</math> is the quantity adsorbed, <math>m</math> is the mass of the adsorbent, <math>P</math> is the pressure of adsorbate and <math>k</math> and <math>n</math> are empirical constants for each adsorbent-adsorbate pair at a given temperature. The function has an asymptotic maximum as pressure increases without bound. As the temperature increases, the constants <math>k</math> and <math>n</math> change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface.

Adsorption lowers surface energy

At the air/liquid interface:

And the solid/liquid interface:

Lowers the surface tension.

Stabilizes dispersions.

Culinary applications

Mayonnaise is a classic example of an emulsion of an oil in water. Howard McGee gives an extensive discussion of how to prepare this well-known condiment:

The surface tension of water makes it highly-favorable for the water and oil to exist in distinct phases. Energy, in the form of vigorous mixing, needs to be added to the mixture to create a dispersion of oil droplets in water. As an order of magnitude estimate, 15 ml of oil can separate into 30 billion drops in the final product! Enthusiastic mixing by hand can achieve droplets on the order of 3 micron, but industrial-grade homogenizers can produce drops less than one micron in size.

As described in the previous section, this process of dispersing the droplets can be made easier with the presence of surfactants, also known as emulsifiers. In mayonnaise, the phospholipid lecithin in the eggs serves this purpose. The proteins in the egg yolks contain separate hydrophobic and hydrophillic regions, which is also effective. Warm, raw eggs yolks are traditionally used since they are more flexible and can flow more easily than their refrigerated or cooked counterparts. The casein in milk and cream are also sometimes used in emulsions.

However, it is not enough to simply create the droplets: something is needed to keep them from coalescing into larger drops. In mayonnaise, the polymers in mustard seeds do the job.

Chocolate is an emulsion of cocoa particles in cocoa butter. Starting in the 1930's, lecithin was used to replace some of the cocoa butter. One part lecithin can lubricate as many cocoa particles as 10 parts cocoa butter. Due to this efficiency, chocolate typically contains only 0.3 to 0.5% lecithin my weight.

Whisky may often be served "on the rocks" to enhance the flavor of the beverage, rather than just to dilute the alcohol. As the ice melts and the liquid becomes more polar, long chain esters and alcohols form micelles, which "masks" their flavor. On the other hand, ethanol becomes more soluble in water as the liquid cools, which causes it to break up existing micelles of flavor molecules. For more information, see the blog post on khymos.org.

Foam

Surfactants lower interfacial tension. This promotes finer dispersions. But they also keep dispersed droplets from coalescing. Surfactant coated surfaces repel each other, and merging of droplets would also require the surfactants to reorganize on the surface. "Thus, it is that two adjacent surfactant-coated droplets can coalesce only on the timescale of years." Thus, emulsions such as mayo or cold cream can have a long shelf life.

Foam is just a dispersion where the solute is air. Foams can be made either by stirring or by lowering the pressure of a gas-saturated solution. The solution becomes supersaturated with gas and begins to bubble. This is what happens with shaving cream or with beer bottles when they are opened.
(Witten p. 197)

Great Experiment: Put some dry ice in soapy water, and you will get soap bubbles rising from the surface!

Gibbs' adsorption isotherm

A derivation by Gibbs gives a relation between the chemical potential of a solute in solution, the surface tension of an interface and the excess concentration the solute at that interface. The interface is considered to be wide compared to the concentration gradients; the excess number of moles associated with that interface is calculated and is expressed as a surface concentration, moles per area:

Adsorption by a solid surface

The adsorption is driven by both strong tail/solid interaction and entropy – the hydrophobic effect.

The adsorption is driven by strong head group/solid interaction.

What would happen to this configuration if the water and oil were alternated at the surface? Would the molecules at the surface just keep flipping?

Adsorption of surfactants on solid surfaces has significant applications. One is detergents, where a dirt particle is surrounded by adsorbed surfactant molecules. Another commercial application is solubility of solid material such as pigment or latex particles in paints. The adsorption of particles from a solution onto a solid surface is described by the Langmuir adsorption equation. At equilibrium, the adsorption rate is equal to the desorption rate, hence:

<math>\frac{d\Theta}{dt} = k_ac(1-\Theta) </math> for adsorption

<math>\frac{d\Theta}{dt} = k_d\Theta </math> for desorptions

Where <math>\Theta </math> is the surface coverage and c is the surfactant concentration. <math>k_a </math> and <math>k_d</math> are the rates of adsorption and desorption respectively. Solving for <math>\Theta</math> at equilibrium yields:
<math>\Theta = \frac{ \frac{k_a}{k_d} c}{1 + \frac{k_a}{k_d}c} = \frac{Kc}{1+Kc}</math>

With <math>K=\frac{k_a}{k_d}</math> being the equilibrium constant.

At the limit <math>K,c >> 1</math> then <math>\Theta \approx 1</math> and the surface is saturated with surfactant.

At the limit of <math>c,K << 1</math>, then <math>\Theta \approx Kc</math> and the coverage is still proportional to concentration.

Adsorption Applications

Adsorbents most commonly exist as spherical rods, pellets or monoliths with hydrodynamic diameters at the order 1 to 10 mm. The properties that characterize them are high abrasion resistance, high thermal stability, as well as small pore diameters. All these characteristics make the exposed surface area greater therefore enable higher surface capacity for adsorption. In addition, adsorbents must also have a specific structure which enables fast transport of the vapors.

Industrial Application: Cryosorption Pumping

The phenomenon of adsorption is very important in vacuum science, and is the physics behind a very important class of vacuum pump. A cryosorption pump (sometimes called simply a cryopump) uses cooled surfaces covered with a material that adsorbs a certain vapor. Because they have no moving parts and require no oil (only a cold surface), cryopumps are a clean, effective, and simple way to achieve a very high high vacuum.

While a flat, smooth surface will cryopump most gases, helium requires a very porous material to be cryopumped. Modern cryopumps use materials with very high surface areas and internals mircopores; common materials are activated charcoal (usually made from coconut, but also made from wood, petroleum byproducts, or bone), porous metals such as copper, or solid argon.

A well-constructed cryopump, cooled down to a few Kelvin, can pump several liters of helium per second per square centimeter, and has a capacity of several Torr-liters per square centimeter. In other words, the adsorbing materials can contain a few hundred times their own volume!

Coconut-charcoal based cryopumps are a common tool in cryogenics labs, and are the vacuum pumps of choice in fusion reactors because of their huge pumping speeds and low contamination.

Adsorption of Virus

Adsorption of virus.

Adsorption is the first step in the viral infection cycle. The next steps are penetration, uncoating, synthesis (transcription if needed, and translation), and release. The virus replication cycle is similar, if not the same, for all types of viruses. Factors such as transcription may or may not be needed if the virus is able to integrate its genomic information in the cell's nucleus, or if the virus can replicate itself directly within the cell's cytoplasm.

Water adsorption and desorption using zeolites

Zeolites are crystalline aluminosilicates, polar in nature, which have a periodic pore network and release water at high temperature. They are produced by hydrothermal synthesis of sodium aluminosilicate (or similar silica source) followed by an ion exchange with a cation (Na+, Li+).

As we mentioned, natural zeolites are highly polar and have the property of adsorbing and desorbing water without damage to its crystal structure. This capability makes them very useful in desiccation processes. In many industrial and commercial applications, zeolites have been found highly effective in controlling moisture levels. They are able to dry the air, remove CO2 from natural gas, remove CO from reforming gas, etc.

Furthermore, ability to adsorb and desorb water without change in its structure, together with a high heat of adsorption, makes zeolites effective and efficient heat energy storage for later use. Unlike other heat energy systems utilizing non-zeolite heat storage which can be very expensive, zeolites provide a low cost, efficient media for heat storage.